Field of the Invention
The invention relates generally to evaluation of joints, and more specifically to mechanical pivot shift measurement to evaluate joints, such as instability in the knee.
Description of the Related Art
The anterior cruciate ligament (ACL) is an intra-articular ligament connecting the lateral femoral condyle to the proximal tibial plateau. The ACL is composed of two distinct functional bundles, the anteromedial (AM) bundle and the posterolateral (PL) bundle. These serve as a primary knee stabilizer and function in concert to limit tibial motion in a manner dependent on the angle of knee flexion. ACL injury is a common occurrence in many sports, with up to 135,000 ACL injuries leading to over 95,000 reconstructions per year in the United States. The ACL is often ruptured in a twisting, non-contact mechanism, with a tear in one of the two bundles occurring 10% of the time, and a complete rupture occurring 90% of the time. In an ACL deficient knee, abnormal kinematics result in medial meniscus tears and degenerative arthritis. The goal of ACL reconstruction is to restore normal knee kinematics, and thereby mechanical/biological homeostasis. Current ACL reconstruction techniques have advanced in the past 20 years to re-establish ACL anatomy, and the immediate outcomes of ACL reconstruction are uniformly good, with 80-95% of patients reporting good to excellent results in many studies. However, despite the short term success of current procedures, the incidence of osteoarthritis is the same in ACL-deficient and ACL-reconstructed knees, indicating that current reconstruction techniques are not adequate in restoring rotational knee kinematics to their native state. Failure to restore rotational stability directly correlates with arthritis progression and poor long-term outcomes.
Given the importance of evaluating rotational stability following ACL-reconstruction, attempts have been made to detect pathological motion patterns in the knee. In a clinical setting, rotational stability of the knee is evaluated with the use of a manual pivot shift maneuver. The pivot shift is a dynamic knee stability test that involves a flexion-dependent combination of an axial force and valgus torque to induce pathologic motion. The presence of a positive pivot shift is predictive of osteoarthritis development, failure to return to previous level of play, patient-reported instability, and poor subjective and objective outcomes following ACL reconstruction.
Despite its correlation with arthritic progression, the pivot shift is highly technique-dependent and variable among practitioners. Consequently, it has poor sensitivity and is difficult to reproduce in a clinical or laboratory setting. Past biomechanical studies have employed various instrumented devices designed to mimic the rotational instability observed in ACL-deficient knees. Since different joint loading combinations induce distinctive kinematic behavior, the results and their interpretation are often inconsistent between studies, making it difficult to determine the effects of graded knee injuries (ligament and meniscus) and different ACL reconstruction techniques, on rotational stability. Thus, improvements in measurement techniques for objectively quantifying knee stability would be valuable to diagnose patients, assess clinical outcomes, and optimize surgical treatment.